Several classes of hydraulic cements are known, such as Portland cement, high-alumina cement, and gypsum which are categorized by their chemical composition and use. For example, Portland cement is commonly used in structural applications and is high in silicate content. In this disclosure the term "cement" shall refer to silicate cement compositions including Portland cement, pozzolanic cements, hydraulic limes, fly ash and natural cements. Cements are used in concrete, grouting and mortar compositions which contain other components such as sand and gravel, or stone.
Cement compositions are rendered useful by the combination of the solid components with water. To obtain a mixture with sufficient workability in order to pour into a mold or form, an amount of water in excess of that required for hydration of the cement components is necessary. During the hardening of the composition, the excess water remains and ultimately, as it evaporates, causes cavities to be formed in the hardened molded structure. These cavities reduce the integrity of the structure and result in a compressive strength which is lower than that theoretically obtainable. It is, therefore, desirable to use the lowest amount of water possible in a cement composition in order to obtain the best compressive strength. Since low amounts of water, while desired for strength, make workability difficult, a compromise is called for.
Additives which cause the strengthening of cement compositions by water reduction while allowing the mixtures to have adequate workability are commonly called water reducers or plasticizers. Those which allow high levels of water reduction are known as high range water reducers or superplasticizers. These superplasticizers act as cement dispersing agents to increase the fluidity of the mix such that less water will produce the same workability, or slump, as a control containing 20% more water.
Advantages of these superplasticizers include (1) the production of higher strength concrete since a lower water/cement ratio is used, and/or (2) easier placement of concrete with more uniform properties because of its improved workability, and (3) a concrete mix of the same workability and strength using less cement in the mix. Lignin sulfonates and sugars are common plasticizers while typical commercial superplasticizers are sulfonated melamine-formaldehyde condensates or sulfonated naphthalene-formaldehyde condensates. In addition, sulfonates of styrene polymers have been described as fluidizing agents for cement compositions.
U.S. Pat. No. 4,164,426 teaches the use of zinc naphthalene-formaldehyde sulfonate in concrete mixes for improved compressive strength. The patented material claimed functions as a superplasticizer but with the added advantage of producing higher strength concrete than normally obtainable, e.g. with the sodium salt of naphthalene-formaldehyde sulfonate.
U.S. Pat. No. 4,071,493 describes alkali metal salts of sulfonated styrene-indene hydrocarbon resins as fluidizing agents for mineral binders.
British Pat. No. 1,588,130 describes a hydraulic cement composition which includes a plasticizer comprising a sulfonated polystyrene. It is taught that the sulfonate is preferably used in the salt form. The inclusion of a water soluble carbonate in the composition apparently improves the plasticizing effect of the sulfonated polystyrene although the sulfonated polystyrene may be used with or without the carbonate.
U.S. Pat. No. 4,076,699 describes a process for the preparation and use of alkali salts of a sulfonated styrene polymer or copolymer as fluidizing agents for mineral binders. The materials exemplified in the patent are salts of sulfonated polystyrene and sulfonated polyvinyltoluene. No examples of copolymers are given.
The period of workability of hydraulic cement mixes containing superplasticizers is considerably shorter than that of normal mixes, generally 30-60 minutes or less before stiffening jeopardizes its proper placement at the worksite. Three methods have been proposed to solve this slump loss (loss of workability with time) problem: (1) delayed addition of superplasticizer at the site in the ready mix truck; (2) repeated doses of superplasticizer to refluidize the mix; (3) addition of a retarder, e.g., a hydroxycarboxylic acid salt. The first two approaches pose difficulties in practice because of variability of temperature, cement type, and content, mixing criteria, etc. The use of retarding admixtures shows some variation in performance. One reference states that sodium heptonate retarder with naphthalene-formaldehyde condensates gives lower loss of workability than with either alone (Edmeades, R. M. and Hewlett, P. C., "Superplasticised Concrete--High Workability Retention," Proceedings of Admixtures Congress, CI 80, Lancaster, Construction Press, 1980, pp. 49-72). Another reference discloses that sodium heptonate with sulfonated melamine-formaldehyde condensates increase initial slump only slightly with a slightly higher slump after two hours compared to a concrete containing the melamine-based superplasticizer alone (Ramachandran, V. S., "Effect of Retarders/Water Reducers on Slump Loss in Superplasticized Concrete", in Developments in the Use of Superplasticizers, American Concrete Institute, SP-68, 1981, pp. 393-407). Finally, sodium gluconate influences slump loss the most, although, at the level disclosed in the above reference, both initial setting times and early compressive strength of mortar were retarded. The suggestion that lower dosages of gluconate would overcome slow setting and low early strengths, possibly with the use of accelerators, is not demonstrated.
A patent (U.S. Pat. No. 3,686,133), earlier than the Edmeades and Hewlitt reference above, discloses the use of a dispersing agent for hydraulic cement which is a combination of a salt of a naphthalene sulfonic acid-formaldehyde condensation product and a salt of gluconic acid. Better flow properties and enhanced strength are said to result.
In another patent (British Pat. No. 1,575,450) a cement composition employs a sodium salt of gluconic acid as a set retarder when used in combination with an .alpha.-hydroxycarbonyl accelerator compound and a carbonate of sodium.
It is the object of this invention to improve both compressive strength of mortars and concretes and lessen slump loss, using known retarders, e.g. gluconic acid salts. Heretofore the gluconic acid salts were not able to influence slump loss without also retarding initial setting times and/or reducing early compressive strength.
It has now been discovered that low levels of gluconate salts when added to concrete mixes in which the sulfonated copolymers of styrene-.alpha.-methyl styrene are dispersed, provide improved and extended workability properties to the mix, along with improved compressive strength properties of the cured concrete.